Tech Stuff on Relays

The fuel pump relay is connected to the tach sensor and uses the impulse to let it know the engine is running. As long as the engine is running the relay stays closed and allows the fuel pump to run. The relay will open with a certain time constant (a second or two) after it receives an impulse unless it receives another one within that time. Essentially it's a timer that is reset according to the tach impulse and if allowed to expire the relay opens. The timing function of the fuel pump relay is a 555 timer chip (available from any Radio Shack for $1.29 or so) which has a common failure mode that causes the time constant to effectively go to zero -- the relay will in this failure mode turn on and off at a frequency equal to the RPM of the engine. The relay cuts out immediately after the tach sensor signal but the rotation of the engine and the fuel in the accumulator keeps it going until the next impulse from the tach which will enable the relay for another pulse. Because the fuel pump delivers far more fuel than the engine consumes and a working fuel accumulator will effectively even out the pulses of fuel being delivered by the pump, I found that my car still ran with the fuel pump relay cycling at the engine RPM speed (I could hear it rattling in the relay compartment). As the RPM's increase, the ability of the mechanical relay and the fuel pump to keep up with the increased cycling rate decreases and the engine becomes starved for fuel.

The relay can aften be repaired for a cost of under $2 by someone familiar with hobby electronics by replacing the 555 timer chip in it. Alternatively, a new Volvo fuel pump relay runs around $30-$35